Combustion control circuit



Jan. 21, 1969 N. A. FoRBEs COMBUSTION CONTROL CIRCUIT Filed May l2, 1966ATTORNEY TRANsFoRMER FIGA United States Patent O 8 Claims ABSTRACT OFTHE DISCLOSURE This invention involves a gas burner control system for aboiler having a plurality of pilot burners. The control system employs arelay switch for each pilot burner. The relay switch lwill be movedbetween two positions, one in response to the operation of theassociated pilot burner and the other in response to the non-operationof that pilot burner. The control system also includes flamedetectingmeans which responds to the condition of the pilot burner to operate theassociated relay switch. All relay switches must be in the non-operationposition at the beginning of a startup, for ignition to be successful,and all pilots must light before the main valve can be opened.

The invention relates to a boiler control system for supervising theoperation of large boilers, and more particularly large atmosphericboilers using multiple flamesensing points and pilots.

Large boiler installations often employ several burners. To insuresmooth ignition multiple pilots are used to light the main burner. Inorder to provide safe vand reliable operation of a gas-fuel boiler, itis necessary to insure proper operation of all pilots, that is, allpilots are supervised.

Therefore, it is an object of the invention that such a system have thefollowing features, namely:

(a) Spark ignited pilot;

(b) All pilots must be proven before the main gas valve delivers gas tothe main burners;

(c) Interrupted ignition (the spark ignition is turned off when allpilots have lit and turns on again if any one pilot goes out);

(d) The main gas valve closes quickly (in two to four seconds) if anyone pilot goes out;

(e) Fail-safe operation in the sense that a component failure must causethe boiler to shut down;

(f) Increased reliability by elimination of certain cornponents found inprior art control systems; and

(g) Greater economy.

Briefly, the invention contemplates a gas burner control system for aboiler having a plurality of pilot burners, with each pilot burnerprovided with a supervisory means. The control system comprises inassociation with each pilot a relay switch operable to a first positionto control a control circuit to prove the related pilot is burning, andoperable to a second position to prove that the related pilot is notburning, and fiame-detecting means responsive to the condition of thepilot for operating the relay switch.

Other objects, features and advantages of the invention will be apparentfrom the following detailed description when read in connection with thediagrammatic circuits in the accompanying drawings, wherein:

FIG. l is a simple schematic diagram to indicate the circuit equivalenceof a flame in a pilot light to a simple diode rectifier with a seriesresistance;

FIG. 2 is a schematic diagram of the flame sensing circuitry containedin each flame detector module for supervising one related fiame, andshows also the circuit relation of the pilot flame detector relay to thecorrespond- Patented Jan. 2l, 1969 ice ing flame detector relays of twoother flame detector modules;

FIG. 3 is a schematic circuit diagram of two of the several pilotsparking gaps; and

FIG. 4 is a schematic circuit diagram of the main control system.

The flame control circuit Each fiame at a pilot is supervised throughthe use of the rectifying properties of a gas flame.

The recognition of flame at a supervised pilot causes a double-poledouble-throw relay inside the flame control module to pull in. Thenormally-closed contacts of one pole of all such relays are connected inone series string and the normally-open contacts of the other pole ofall such relays are connected in another series string. These two seriesconnections are used by the main control circuit, described below, tocheck that all flame detector relays are dropped out at the beginning ofa trial for ignition, and that all flame detector relays are pulled in,indicating that all pilots are lit, before the main gas valve is opened.

Referring to FIGURES l and 2, a 230-volt, 60-cycle AC voltage is coupledthrough capacitor C1 to the flame rectifier RF which is shownschematically as embodying a diode De and a resistor Rs and sometimesreferred to as the flame rectifier. When no liame is present, the flamerectifier RF is essentially an open circuit with an AC voltage acrossit. When a flame is present, the flame rectifier RF behaves like a diodeDe with a very high series resistance RS, approximately l0 megohms, andwith appreciable leakage. The cathode of the flame rectifier RF isnecessarily grounded, because the rectifying properties of a flame andthe polarity of the flame rectifier RF depend on the relative area ofthe ground electrode and the probe. In spite of the imperfectrectification by the fiame, enough rectification occurs to cause thejunction J1 (see FIG. 2) of the flame rectifier RF and capacitor C1 tobe charged negatively by approximately volts. The voltage at thisjunction is therefore the algebraic sum of a 230-volt sine wave and a DCvoltage of -100 volts.

A resistor R1 and a capacitor C2 in series bridge the flame rectifierRF. The junction J2 of the resistor R1 and capacitor C2 is connected tosubsequent series circuit including resistor R2 and capacitor C3. Thejunction I3 of resistor R2 and capacitor C3 is connected via a neon lampN1 to a relay control circuit containing silicon controlled rectifierSCR1 having an anode A, a cathode C, a gate G and it is controlled by adiode D1 and a resistor R3, to operate the flame relay coil IRL from a24-volt, 60cycle supply source.

Resistor R1 and capacitor C2 of FIG. 2 form a filter which removes allbut 25 Volts of the 230 volt AC component from the flame rectifiervoltage, while leaving the DC component relatively unattenuated. Thevoltage at the junction J2 of resistor R1 and capacitor C2 is thereforethe algebraic sum of a 25 volt sine wave and a DC voltage ofapproximately -100 volts. The 25 volt AC component in this waveform,when correctly phased with anode-to-cathode voltage across siliconcontrolled rectifier SCR1 ensures that there is no relay chatter atfiame signals just large enough to fire neon lamp N1.

Resistor R2, capacitor C3, and neon lamp N1 form a discharge path acrosscapacitor C2. When capacitor C2 is charged, neon lamp N1 discharges in aseries of short pulses several hundred times a second. In this processresistor R2 serves two purposes: (l) it ensures that neon lamp N1discharges in many closely spaced pulses at a rate determined by thetime constant of resistor R2 and capacitor C3, instead of a few widelyspaced pulses, (2) together with capacitor C2 it provides a delayedrelease circuit so that loss of flame recognition does not occur morerapidly than in two to four seconds. Capacitor C3 determines both thefiring rate (about 350 pulses per second) and the peak current perdischarge pulse (about two milliamperes). The discharge itself takesabout seven microseconds.

Current through neon lamp N1 is used to turn on silicon controlledrectifier SCR1. However, instead of driving the gate G of switch SCR1directly in the usual manner, the cathode C is driven. The reason forthis connection is that the gate and cathode terminals of switch SCR1must be bridged with a low resistance (1,000 ohms), thereby preventingthe leakage current through silicon controlled rectifier SCR1 frompossibly turning it on. If the gate were driven directly, this resistorwould reduce the system current sensitivity. Also the resistor would notbe failsafe, since if it failed because it opened it would not shut downthe burner. In the schematic diagram of FIG. 2, this resistor R3 isconnected between the gate G of switch SCR1 and ground. In this positionresistor R3 prevents leakage current from firing SCR1, yet does not haveas much effect on the sensitivity of the silicon controlled rectifierSCR1 as a gate drive system would have. In practice, the cathode drivecircuit shown has typically a 150 microampere sensitivity when using asilicon controlled rectifier with a basic sensitivity of 30microamperes, the difference being due to current cancellation in thecathode drive circuit by transistor action within the silicon controlledrectifier before it fires. A gate drive system with 1000 ohms across thegate to cathode terminals would by comparison have about a 700microampere sensitivity.

Diode D1 has two functions: (l) when discharge current from capacitor C3flows through neon lamp N1, reverse voltage appears across diode D1,diode D1 blocks the voltage, and the discharge current flows out ofcathode C and into the gate G of silicon controlled rectifier SCR1, thusturning silicon controlled rectifier SCR1 on, and (2) when siliconcontrolled rectifier SCR1 fires, diode D1 conducts in the forwarddirection and provides an easy path for cathode current.

Diode D2 is a free-wheeling diode across the coil 1RL. Even though thecoil 1RL is driven by a half-wave voltage, the current in the coil 1RLis relatively smooth because the inductance of the coil 1RL causescurrent to continue flowing through the coil 1RL and through diode D2long after the voltage across coil 1RL has fallen to zero.

Relay coil 1RL is associated with two contacts, as shown in FIG. 2, andthe assembly is identified as flame relay 1R. The normally open contactslRCO, 2RCO, 3RCO of all such flame relays 1R, 2R, 3R, etc., areconnected in series as shown, so that a connection is made in the maincontrol circuit when flame is established at all supervised pilots. Thenormally-closed contacts IRCC, 2RCC, 3RCC, etc., of all such llamerelays are also all connected in series so that the main control circuitcan check that all such flame relays have dropped out 'before thebeginning of a trial for ignition. Each flame relay 1R, 2R, 3R istherefore checked against sticking in either a closed or an openposition.

Fail-safe analysis Although the manner in which the circuit is madefailsafe is implied in the foregoing, the subject of safety controlcircuits being fail-safe is so important that a separate fail-safeanalysis is presented below. The definition of fail-safe, as the term isused here, is `as follows: an electronic component fails safe if eitheropening or shorting it causes the ow of fuel to cease, eitherimmediately or at the beginning of the next ignition cycle.

If capacitor C1 of FIG. 2 opens, no voltage can be applied to the flamerectifier RF and there is no flame recognition signal to fire neon lampN1. If capacitor C1 shorts, the voltage across the llame rectifier RFhas no DC component (even though the current through it does), so thatagain there is no flame recognition signal.

If resistor R1 shorts, then capacitor C2 is connected directly acrossthe flame rectifier RF and the voltage divider effect of capacitors C1and C2 causes the voltage applied to the flame rectifier RF to be so lowthat the flame recognition signal is insuflicient to fire neon lamp N1.lf resistor R1 opens, the flame recognition signal is prevented fromreaching neon lamp N1.

If capacitor C2 shorts, the flame recognition signal is shorted toground and cannot fire neon lamp N1. If capacitor C2 opens, itsfiltering action ceases and the large AC component in the flamerecognition signal fires neon lamp N1 continuously. Flame relay 1R nowpulls in permanently. At the beginning of the next ignition cycle, thecontrol system checks the flame relays `in all the flame detectormodules to make sure that they have all dropped out. If one flame relayhas not dropped out, no fuel can be delivered to either the pilot gasvalves or the main gas valve, and the trial for ignition stops.

If resistor R2 opens, the flame recognition signal is prevented fromreaching neon lamp N1. If resistor R2 shorts, the frequency at whichneon lamp N1 discharges falls so low that the gate G of siliconcontrolled rectifier SCR1 does not receive positive drive every cycle,and flame relay 1R does not pull in.

If neon lamp N1 opens, switch SCR1 cannot be turned on and flame relay1R does not pull in. If neon lamp N1 shorts, then the gate G of siliconcontrolled rectifier SCR1 receives a small continuous current of severalmicroamperes instead of a series of pulses of approximatelymicroamperes. Under these conditions, the gate drive is too low to firesilicon controlled rectifier SCR1, and llame relay 1R does not pull in.

If capacitor C3 shorts, there is no voltage available to fire neon lampN1, silicon controlled rectifier SCR1 does not fire, and llame relay 1Rdoes not pull in. If capacitor C3 opens, neon lamp N1 fires morerapidly, but with such low current that silicon controlled rectifierSCR1 does not fire, and flame relay 1R does not pull in.

If resistor R3 opens, no current can flow into the gate G of siliconcontrolled rectifier SCR1, so that switch SCR1 does not fire, and flamerelay 1R does not pull in. If resistor R3 shorts, then the initialbuildup of cathode current in silicon controlled rectifier SCR1(transistor action) cancels the gate drive, so that silicon controlledrecifier SCR1 does not fire and flame relay 1R does not pul in.

If diode D1 shorts, the cathode of silicon controlled rectifier SCR1 isshorted to ground, silicon controlled rectifier SCR1 does not fire, andrelay 1R does not pull in. If diode D1 opens, there is no path to groundfor the cathode current of silicon controlled rectifier SCR1, so thatflame relay 1R does not pull in.

If diode D2 shorts, the coil 1RL of flame relay 1R is shorted and theflame relay does not pull in. If diode D2 opens, the current in relaycoil 1RL is made to pulsate so that flame relay 1R chatters, but doesnot pull in.

If silicon controlled rectifier SCR1 shorts, flame relay 1R pulls incontinuously, causing a shutdown at the next trial for ignition.

The main control circuit The main control circuit is used only once in aboiler installation regardless of the number of supervised pilots.

The main control circuit combines information from all the kflamecontrol modules, so that all flame relays must drop out before a trialfor ignition can begin, and all llame relays must pull in before gas isdelivered to the main burners.

A schematic diagram of the main control is shown in FIG. 4.

The main operating lcomponents are indicated as Controller CR, forappropriate initiating or control switching; PV, for pilot gas valve;IG, for ignition transformer;

MV, for main gas valve; SS, for a safety switch that is essentially athermallyoperated time delay relay switch; TL, a temperature limitcontrol; LR, a load relay coil LRL associated with contacts LR1 and LRZ;and a llame relay coil FRL associated with contacts FR1, FRZ, FR3 andFR4.

When all the individu-al flame relays 1R, 2R, etc., in the respectiveflame detector modules of FIG. 2 have dropped out, a series circuitthrough the normallyeclosed back contacts 1RCC, ZRCC, 3RCC of one poleon each of all these relays make a through closed connection, asindicated in FIG. 2. This closed series connection is shown also in FIG.4.

Similarly, when all the flame relays of the respective pilot flamedetectors are energized, indicating all pilot flames are lit, a seriescircuit through the normally-open contacts 1RCO, ZRCO, 3RCO, of anotherpole on each of all the flame relays, is closed to make a closed circuitconnection. These contacts are shown in FIG. 4.

Under normal conditions, therefore, when the controller CR calls forheat, the load relay LR is energized initially by current passing viacapacitor C4, the closed contacts of safety switch SS, the heaterwinding of safety switch SS, the closed back `contact FRZ, the seriesconnected contacts lRCC, 2RCC, 3RCC and coil LRL. If any of thesecontacts is open relay coil 1RL will not be energized and the trial forignition will cease.

If everything is normal, load relay coil LRL is energized contact LR1shorts out the series connection of contacts FR2 1RCC, ZRCC and 3RCC sothat these four contacts can subsequently open. Also, on the 120 voltline, contact LRZ energizes the pilot gas valve PV and the ignitiontransformer IG.

In `a normal ignition, the pilots will all light, but not necessarily inunison. The first pilot to light opens a contact in the series contactcircuit 1RCC, 2RCC, 3RCC, and the last pilot to light closes the seriescontact circuit 1RCO, ZRCO, 3RCO, energizing relay coil FRL, openingcont-act FRZ, and closing its `contact FRI.

Up to the instant that front contact FR1 closes, all the current forrelay coil LRL has been owing through the heater winding of the safetyswitch SS. If ignition of all pilots is not confirmed by the closing offront contact FRI within (typically) seconds, then safety switch SS,which is essentially a temperature-compensated thermal circuit breaker,operates, opening its back contact SS1 and terminating the trial forigntion. However, if front contact FR1 vdoes close before l5 seconds,then the heater of safety switch SS is shorted and the trial forignition is successfully completed.

Successful lighting of all the pilots, as indicated by the energizing ofrelay coil FRL, causes the main gas valve MV to be energized throughclosing contact FR3, and causes the ignition IG to be interrupted byopening contact FR4, `since all pilots are now ignited. The boiler isnow operating normally.

If any one of the pilots such as the pilot associated with relay 1Rextinguishes for any reason, then relay coil IRL becomes de-energizedand the series circuit 1RCO, ZRCO, 3RCO opens, relay coil FRL isdeenergized, con tact FRI opens, the heater of safety switch SS isre-energized, the ignition IG is 1re-energized, and the main gas valveMV is de-energized. The circuit is now back in the trial for ignitionmode, and if all the pilots do not now light, the safety switch SS willcause a safety shutdown as before.

For successful operation of the main control circuit, the current inrelay coil LRL must not change appreciably when the heater of safetyswitch SS is shorted. One conventional way to achieve this objective isto design relay coil LRL with an inductance that is high compared toboth its own resistance and the resistance of the safety switch heater,so that shorting the safety switch heater upon flame recognition doesnot cause a large change in current through the relay coil LRL. However,such a design requires a large wire size, resulting in a large coilbobbin and a large core. Such a relay is wasteful of materials by normalstandards, iand hence is a special design. Another conventional approachto this problem is to make relay coil LRL close a third contact, and usethe extra contact to connect the junction of safety switch SS andcontact LR1 (FIG. 3) to a tap on a 24-volt transformer, so that relaycoil LRL receives the correct voltage under normal running conditions.This approach solves the problem of current increase through relay coilLRL when the safety switch heater is shorted, but raises the problem ofprocuring a three-pole relay with spacings suicient to meet ULrequirements. In the present design, a constant-current source has beencreated by the use of capacitor C4, with the result that the current inthe coil LRL changes only slightly when safety switch SS is shorted, sothat a special design for relay coil LRL is not needed. Fuse F1 protectsthe circuit against a short in capacitor C4.

Ignition circuit The ignition circuit developed for this control systemis shown schematically in FIG. 3.

The objective of this ignition circuit is to permit multiple pilots tobe ignited from a single ignition transformer T, having a primarywinding Tp for receiving the AC ignition voltage and a secondary windingTs. This objective is achieved by placing a small capacitor, C5 in FIG.3, rated at 630 pf. and l0 kv. in series with spark gap SG1 and asimilar capacitor C6 in series with spark gap SG2. To limit the current,resistor R4 is placed in series with capacitor C5 and resistor R5 isplaced in series with capacitor C6. In practice, as the terminal voltageof the ignition transformer rises, the gaps fire in a random manner.When a gap such as gap SG1 fires, a pulse of current passes across thegap until the corresponding capacitor C5 in series with the gap ischarged. At some later instant, another gap SG2 fires, charging itsseries capacitor C6 and causing the terminal voltage of the transformerto dip momentarily. This dip in voltage causes the first capacitor C5 todischarge into the second capacitor C6, intensifying the spark in thesecond gap SG2. As many as ten spark gaps have been operated in thismanner from one ignition transformer. The analysis presented above isconfirmed by the fact that as each new spark gap was added, the energyin the gaps increased. The effect of resistors R4 and R5 is to reducespark gap erosion, and to limit the rate of rise of transient voltagesacross transformer T.

The capacitors C5 and C6 are fail-safe because if one capacitor such asC5 opens, then the spark gap SG1 in series with the capacitor C5 willnot be energized and the associated .pilot will fail to ignite, while ifone capacitor shorts, then the spark gap in series with the capacitorwill be the only one to be energized and all the others will fail tolight. In either case, since all the pilots fail to light, the controlcircuit goes through a safety shutdown as shown in a previous analysis.The resistors are failsafe only against opening; therefore they wouldhave to be of a type that is not likely to short.

Thus, by the system circuitry shown, a simple economical and safeignition control system is provided for a multiple-pilot furnace. Itwill be understood, of course, that variations or re-arrangements of thecircuitry and components may be made without departing from the spiritand scope of the invention, as set forth in the claims.

What is claimed is:

1. A gas burner control system for a boiler having a plurality of gasburners, with each burner provided with a pilot light, said controlsystem comprising, in association with each pilot light, a relay switchoperable to a iirst position to control an indicating circuit toindicate that the related pilot light is burning, and operable to asecond position to indicate that the related pilot light is not burning;dame-detecting means responsive to the condition of the pilot flame foroperating the relay switch; said relay switch including an operatingcoil; a silicon controlled rectifier switch for controlling theenergization of said operating coil; and a pulsing circuit as part ofsaid flame-detecting means and operable at a high pulsing rate tocontrol the operation of said silicon controlled rectifier switch.

2. A gas burner control system, as in claim 1, in which saiddame-detecting means includes a circuit including the pilot light fiameand a capacitor to establish a voltage-divider circuit with a variablevoltage across the pilot ame; the pulsing circuit including acapacitorresistor circuit connected to be energized from said variablevoltage and to have a relatively short time constant; a gas dischargedevice for discharging the capacitor of said capacitor-resistor circuitand for generating a rapid sequence of short time pulses; and meanscontrolled by said pulses for controlling the operation of said relayswitch.

3. A gas burner control system, `as in claim 2, in which said meanscontrolled by said pulses includes a circuit including the siliconcontrolled rectifier.

4. A gas burner control system, as in claim 3, in which said siliconcontrolled rectifier is provided with an anode, a cathode and a tiringelectrode; and said pulsing discharge device is connected to the cathodeof said silicon controlled rectifier to cause rapid firing and rapidenergization of said silicon controlled rectifier for establishingsubstantially continuous energization of said operating coil for saidrelay switch.

5. A gas burner control system, as in claim 1, in which ignition meansare provided to ignite each pilot gas stream; and said rel-ay switches,when all are operated to their respective first positions, complete acircuit to indicate that all pilot flames are burning effectively; andmeans are provided to be responsive to said circuit for 8 operating amain valve to said plurality of gas burners When said pilot flames areall ignited.

6. A gas burner control system, as in claim 5, including a main valvefor controlling a main gas stre-am to the several burners; and meansresponsive to said circuit that indicates all pilot flames are ignited,for controlling the operation of said main valve.

7. A gas burner control system, as in claim 6, including means forde-energizing said ignition means when all the pilot flames are ignited.

8. A gas burner control system, as in claim 1, in which ignition meansare provided to ignite each pilot light gas stream, said ignition meanscomprising an ignition transformer including a primary winding forreceiving an AC ignition voltage and a secondary winding, and aplurality of ignition circuits, each connected in parallel with saidsecondary Winding, each of said ignition circuits comprising spacedignition electrodes and a capacitor and resistor connected in series.

References Cited UNITED STATES PATENTS 2,127,445 8/1938 Hardgrove 158-282,216,534 10/1940 Kirk 15S-28 2,410,524 11/1946 Richardson et al. 15S-283,150,709 9/1964 Bolmgren 158-28 3,266,026 9/1966 Plambeck 158-28 X3,301,307 1/1967 Nishigaki 158-28 3,318,358 5/1967 Potts 158-125 X3,238,423 3/1966 Giuffrida 328-6 X 3,348,104 10/1967 Zielinski et al.317-130 FREDERICK KETTERER, Primary Examiner.

U.S. Cl. X.R.

